1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatuses for field-of-view limit detection for optical devices such as quadrant-array detector systems.
2. Description of Related Art
Semi-active laser (SAL) seeker systems are usually subject to specific field of view requirements. These requirements will vary by system and application, but in general, they establish minimum, nominal, or maximum values for the field of view limit. In the case of a maximum limit, a method must be employed to ensure that no projectile guidance commands are issued when the target is beyond the specified field of view limit. Information about the position of the target relative to the field of view limit is required to comply with the requirement. The present invention provides this information, and allows the guidance processor to enable and disable guidance commands in accordance with the field of view requirement.
Existing solutions to this problem include placing a fixed physical barrier at and beyond the field of view limit, and terminating the active area of the quadrant detector at the field of view limit. Both of these solutions function by reducing the amount of signal that reaches the quadrant detector. The first method “shades” the detector through the use of an undersized aperture. The second reduces the amount of solid angle taken up by the quadrant detector within the optical field of view of the overall detector design.
Existing solutions are based on removing signal from the quadrant detector as the target moves out of the field of view. The simplicity of the quadrant detector permits accurate measurements of target position only when the image of the target spans more than one photocell. With the target near the edge of the field of view, the target image will generally be isolated to one photocell, or between two photocells with position detection in a direction parallel to the edge of the field of view. Under those conditions, the only information about target position provided by the basic quadrant detector is that it lies somewhere between the measurement region in the center and the outer edge of the detection region. The present invention provides positive indication that the target is either entering or exiting the seeker's field of view. This information may be used in enhanced target tracking and guidance algorithms. These advantages may be provided at a low cost using proven technology.